In an optical communication system, an optical IQ modulator based on a Mach-Zehnder Modulator (MZM) is usually adopted to generate a high-order-modulated optical signal. However, factors, such as temperature, will affect stability of the optical IQ modulator, so that an offset occurs in a quiescent operating point of the optical IQ modulator, resulting in deterioration of system's performance. In order to guarantee that a signal quality does not affect the system's performance, it is necessary to monitor and control a DC bias voltage of the optical IQ modulator, so that the optical IQ modulator operates at an optimal quiescent operating point. Furthermore, in order to guarantee mutual orthogonality between an I-branch signal and a Q-branch signal, it is also necessary to adjust a bias voltage of an optical phase shifter to an optimum value. Many studies have been performed on such issues. For example, for a single carrier system based on Quadrature Phase Shift Keying (QPSK) and 16-Quadrature Amplitude Modulation (16-QAM), a method for monitoring and controlling the bias voltage thereof based on an asymmetric dither technology has been proposed. For the single carrier system, the bias voltage control can also be performed by using differential phase information.
With increasing popularity of 100G optical communication technologies, an Optical Orthogonal Frequency Division Multiplexing (O-OFDM) technology has been widely used. However, the study on the bias voltage control for the OFDM system is currently insufficient, and only a few documents disclosed controlling the bias voltage by detecting a power of an output optical signal of the OFDM modulator. Most of existing methods for single carrier bias voltage control and OFDM bias voltage control adopt a manner of power detection. However, actual researches show that although these methods are feasible in theory, the power of the optical signal does not change obviously in an area around the optimal quiescent operating point, and device noises are apt to cause a failure of controlling the bias voltage at an optimum point, leading to a loss of the system performance.